1. Technical Field
The present invention relates to a switching circuit for connecting a first circuit node to either a second or a third circuit node in relation to the potential on the latter node, and more particularly for controlling the potential of the insulation region of an integrated circuit in relation to the potential of the substrate.
2. Description of the Related Art
Consider a circuit in which there is a control device for driving a power device such as an NPN transistor or Darlington, which has a resonant load connected to its collector. When the control device switches the power device, the potential on the collector of the power device can assume negative values. This does not represent a problem in the case of hybrid circuit, with a power device physically insulated from the integrated circuit which forms the control device. However, such negative voltages can be extremely harmful in the case wherein the control device and the power device are integrated on a single silicon chip in a vertical-type structure.
Under normal operating conditions, the insulating region (or P-well) of that part of the integrated circuit used to fabricate the control device is connected to the lowest potential available. In practice, this is ground potential. During operation of the power device, its collector is at a positive voltage. In an integrated circuit device, the collector is the substrate of the integrated circuit. During operation, parasitic transistors which are formed in the integrated circuit are switched off.
If for any reason, for example when the control device executes switching operations, the substrate goes below ground by an amount more than the conduction threshold, the parasitic transistors go into conduction and drain current away from the control device. This causes a malfunction of both the control device and the power device.
To eliminate this drawback, it is in theory sufficient to connect the insulation region of the power device to a negative voltage equal to the maximum below ground value reachable by the substrate. However, negative power sources are not always available to the integrated circuit, and the lowest available potential is normally that of ground, constituted by the casing.
A circuit which allows this drawback to be overcome is described in patent application Ser. No. 89/16144, filed in France on 29 Nov. 1989 in the name of SGS-THOMSON MICROELECTRONICS and SIEMENS AUTOMOTIVE. In such application, a dynamic insulation circuit for integrated circuits is described. Such circuit is usable in an integrated circuit of the type described above, comprising a control device driving a power device connected to a resonant load. This circuit provides that use is made of MOS devices to fabricate a first stage suitable for generating a first voltage level relative to the value of the voltage assumed by the integrated circuits' substrate. It further includes a second stage which has a first and a second transistor acting as a switch. This switch is suitable for connecting the insulation region to either the ground or the substrate, depending upon the values of voltage assumed by the substrate itself.
The described circuit, however, is very complicated due to the large number of circuit components required, and due to the manner in which the conductive state of the second transistor is obtained. Additionally, the switching threshold of the second transistor with the switching function is obtained relative to the substrate's voltage.